1. Field of the Invention
The present invention relates to a method of manufacturing an active matrix type liquid crystal display, and particularly to a method of forming interconnect films such as a lower electrode/interconnect film, gate electrode film, scanning electrode/interconnect film, and data signal row electrode/interconnect film.
2. Description of the Related Art
A Liquid crystal display (hereinafter, referred to as LCD) is advantageous to be thinned more than conventional cathode-ray tubes and has a possibility of obtaining a high resolution image, and in recent years, it has been extensively used. The LCD is classified into an active matrix type and a simple matrix type. The former has a structure in which a switching element of two or three-terminals is incorporated in each pixel; and the latter has not such a structure. Conventionally, the latter has been mainly used; however, in recent years, to achieve high resolution image and to increase the size, the active matrix type LCD has come to be widely used.
The above-described switching elements have been variously proposed. However, there has been a trend in the needs and in the development toward a large sized color liquid crystal panel, and a TFT (Thin Film Transistor) having three-terminals has come to be widely used as the switching element because of the merit in obtaining a high gradation required for full-coloring.
As shown in FIG. 3, the TFT is a field effect type transistor having a structure in which a gate electrode 4, a gate insulating film 5, a semiconductor film 6 and a source/drain electrode 7 are sequentially layered. The gate insulating film 5 is commonly made of SiN.sub.x, and it requires the reliability in the interlayer insulation. To ensure such reliability, the gate insulating film 5 must be less in pin-hole and is densified. However, in the above SiN.sub.x, the pin-hole is difficult to be perfectly removed, and local defects due to refuse upon film formation are often generated. On the other hand, the gate insulating film 5 serves as the interlayer insulating film at the portion where a signal interconnect film intersects a gate interconnect film, and it particularly requires the reliability in the insulation. In view of the foregoing, as shown in FIG. 4, the gate electrode 4 made of Ta, Al, MoTa or the like is anodic-oxidized, to form an anodic oxidation film 4f, and a SiN.sub.x film 8 is layered thereon. Thus, the layer composed of the anodic oxidation film 4f and the SiN.sub.x film 8 is used as the gate insulating film 5. Namely, the gate insulating film 5 having a double structure composed of the lower portion of the anodic oxidation film 4f and the upper portion of the SiN.sub.x film 8 has come to be been mainly used. The anodic oxidation film 4f is commonly made of Ta.sub.2 O.sub.5 or Al.sub.2 O.sub.3 in terms of the withstand voltage, strength and structural homogeneity (pin hole free). The layer 9 composed of the gate electrode 4 and the anodic oxidation film 4f is called a gate interconnect film.
In the active matrix type liquid crystal display (hereinafter, referred to as AMLCD) using a TFT, the anodic oxidation film 4f is made of Ta.sub.2 O.sub.5 or Al.sub.2 O.sub.3, as described above, and thereby the gate electrode 4 is made of Ta or Al. In this case, the use of Ta and Al brings about the following advantages and problems.
In the former, the Ta.sub.2 O.sub.5 film has a high dielectric constant and a high withstand voltage, and further it is excellent in smoothness leading to the precise control of the film thickness. However, in the case where the Ta film of the gate electrode 4 serves as the scanning electrode interconnect film, there occurs a problem in increasing the delay time of a gate bus line because of a large resistivity, as a result of which the Ta film cannot be used for a large size LCD.
In the latter, since Al has a low resistivity, it is effective for preventing the delay time of a gate bus line. However, the Al.sub.2 O.sub.3 film is inferior in the withstand voltage to the Ta.sub.2 O.sub.5 film, and has a difficulty in controlling the film quality upon anodic oxidation. Consequently, the use of the Al film as an interconnect film or an electrode film of the TFT-LCD causes a problem in reducing the reliability of the TFT, that is, of the AMLCD resulting in the poor yield.
A technique for improving the anodic oxidation characteristic (withstand voltage of anodic oxidation film) of the Al film has been proposed in Unexamined Japanese Patent Publication Nos. HEI 5-297389 and HEI 5-341315, wherein the Al film is added with Ti and Ta in specified amounts. As compared with the pure Al interconnect/electrode film, the Al alloy interconnect/electrode film is excellent in the anodic oxidation characteristic, and thereby it has a high withstand voltage. However, the Al alloy interconnect/electrode film is insufficient in the withstand voltage compared with the pure Ta interconnect/electrode film, and is required to be further improved in the anodic oxidation characteristic. On the other hand, in the Al alloy film added with alloy elements such as Ti and Ta, the resistivity is increased linearly with the added amount, and accordingly, it is not optimized from the viewpoint of shortening of the delay time of a gate bus line.